Timo Peltola, The era of LHC-seminar, 2012
TCAD SIMULATIONS FOR SILICON DETECTOR UPGRADE AT LHC
T. Peltola1) 1)Helsinki Institute of Physics, CMS Tracker Project.
1
Outline: • Introduction/Motivation • CMS Tracker • TCAD simulations
o Physics of silicon detector o Device structures o Simulations of static characteristics o Simulations of particle detection
• Summary
Timo Peltola, The era of LHC-seminar, 2012 2
Si-detector signal evolution as a function of fluence [1].
LHC sLHC
sLHC LHC Strips
Pixels
Introduction
Upgrade: LHC → sLHC • L= 1035cm-2s-1 with an event rate of 40MHz
– ∫L = 3000fb-1 • Challenges for tracker:
– Higher radiation hardness – High occupancy → higher granularity – Reduce material budget
Currently used materials, p-in-n FZ for strip and n-in-n FZ for pixel sensors are not reliable at sLHC fluences.
Timo Peltola, The era of LHC-seminar, 2012 3
Approaches for radiation hardness upgrade • P-type material or more exotic materials like SiC or diamond → Only partial benefits for charged hadrons • Detectors in cryogenic temperatures → Difficult engineering • Special detector geometries (e.g. 3D) → High cost • Thin detectors → Small signal, difficult production, high cost • Forward-biased mode (Current Injected Detector [CID])
Evolution of collected charge as a function of fluence for CID and reverse biased detector [2].
CMS Tracker sensor upgrade:
Si Devices Simulation WG • Modeling of devices & study of static and dynamic properties • Comparison w/ experimental results RD50 Detector Simulation
group • Modeling of the dependence of CCE on the detector operational conditions (e.g. T, V, Ф)
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• Paths of charged particles are recorded by finding their positions at key points. • Position accuracy ~10 µm. • Design: Si microstrip detectors (MSSD) surrounding the core of Si pixels.
TOB
TIB
Pix
TID
TEC
300 cm
130 cm
|η| < 2.5
TOB
TIB
• 65M pixels
• 10M detector strips read by 80k microelectronic chips [3].
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TCAD simulations
• Topics [4] – Modeling of devices
• Sensor and detector (Pixel, Strip) • 2D/3D
– Study of static and dynamic device properties • Doping conc., device configuration scans • Transient current scans
– Real devices • Comparison with Lab measurement results • Comparison between simulators
– Device design • Guidelines for pre-processing design optimization
– Study of • Charge Collection and S/N • Degradation of detector performance
• Technology Computer-Aided Design (TCAD): Using simulations to develop and optimize semiconductor processing technologies and devices. • Solves diffusion and transport equations for discretized geometries. • Deep physical approach → substitute resource-consuming test wafer runs by TCAD simulations. • Two main branches: process and device simulation
p+ implant
n-type substrate
Simulation framework
Synopsys TCAD E-2010.12:
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trap AD NNnpqP
Electrostatic potential from the solution of Poisson eq. [5]:
Continuity eqs. describing charge conservation:
t
nqqRJ netn
t
pqqRJ netp
Physics of a silicon detector
charged particle track
• P = ferroelectric polarization
• ρtrap= charge density of traps &
fixed charges
• Rnet= net recombination rate
• n, p from quasi-Fermi potentials
w/ Boltzmann statistics
Operating principle
of a silicon particle
detector:
- +
bNaN AD
0
2
2
dqNV
eff
fd
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Steps to create a Synopsys-TCAD simulation
1. Device geometry, doping levels, meshing
2. Tool flow, command files (Tcl)
4. Plotting & analysis
of data
3. Parameters, experiments, simulation run
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Device structures
X [um]
p-in-n MSSD 2D design
Backplane: n+
n-
Electrode (anode)
n-
p+
Front surface with two strips • 10/60µm width/pitch
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Mesh design
Al 0.5µm
Biasing electrode
Charge collecting electrode SiO2 0.3µm
n-
p+
p-n junction
Edge of the depletion region
Timo Peltola, The era of LHC-seminar, 2012
Pixel detector design with 18 guard rings (GR) • innermost GR 100µm • 16 GR’s 10/10 width/gap • outermost GR 100µm
12
Pixel n-in-n
Pixel and strip structures
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3D design of the MSSD with deep diffusion 120/320µm.
MSSD design with deep diffusion: Pitch=120µm, Implant width=16µm, Al width=29µm.
120/320μm MSSD p-in-n
200/320μm MSSD p-in-n
320μm MSSD p-in-n
Doping profiles at x=center of the implant
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120/320μm MSSD n-in-p
p-spray
Doping profiles: n+, p-spray
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Simulations of quasistationary characteristics
Finding the simulated full depletion voltage (Vfd):
Simulated current-voltage (IV) for p-in-n MSSD:
Experimental value of Vfd ~260V [6].
Vfd
MSSD 300μm p-in-n: 150V voltage ramps 0V 150V 300V
450V 600V
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MSSD 120/320μm p-in-n: 150V voltage ramps
Magnitude of E ~2 orders higher on the edges:
0V 50V 100V
150V
200V
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Two p-in-n MSSD detectors with equal active area size but different total thickness:
Difference in leakage current 10% at 300 V. Full depletion not as effective
in DF detector!
Thinned Deep diffusion (DF)
Simulated IV
Simulated capacitance-voltage (CV):
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Experimental results by Karlsruhe Institute of Technology (KIT), 2011.
Simulated CV/IV for deep diffusion strip detectors.
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0V 0V
0V
Pixel n-in-n
0V
Pixel n-in-n
Pixel n-in-n Pixel n-in-n
Behavior of depletion region and amplitude of E as a function of the position of grounded GR:
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0V
Pixel n-in-n
1 2 3 4 5
Assignement: How many GRs necessary around the active area?
Pixel n-in-n
16 floating GRs
0V 0V
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6e12
1e16 1e15
1e14 1e13
120/320μm n-in-p MSSD p-spray concentration scan : When p-spray concentration is increased 6e12 → 1e16 cm-3 , depletion region no longer extends to p+.
Expected behavior: Oxide charge in SiO2/Si interface needs to be defined as in real
detector.
I [A]
V [V]
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Simulations of particle detection
The generation rate caused by an alpha particle with energy E [5]:
2
2
121
2
2
222
2
1exp
2
1
2
1exp
2),,,(
ucec
w
wv
s
tt
s
atwvuG u
t
m
if , and by: 31 u 0),,,( twvuG ,if . 31 u • u = coordinate along the particle path • v, w = coordinates orthogonal to u • tm = time of the generation peak • α1 = Bragg peak max.
Transients produced by 5.5 MeV α-particle
injection.
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Charge injection scan of HPK 120/320μm p-in-n
I [A]
t [s]
MSSD 300μm p-in-n MSSD 300μm p-in-n backplane
Scans agree with experimental results of V. Eremin et al. [NIMA 500 (2003) 121-132]
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Summary
• Two simulation WGs founded 2011-12 at CMS • Structures: Strip (2D/3D)- and pixel detector 2D • Detector characteristics
o Doping concentration scans o Device configuration scans o Extraction of full depletion voltage from CV/IV
• Comparison with Lab measurement results o CV/IV properties agree mostly with experiment o Absolute values of CV/IV results need adjusting o Transient scans agree with experimental results
• Next step: o Device processing o Radiation induced degradation of detector performance
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References
[1] Karl-Heinz Hoffmann - CMS Tracker Collaboration (2010), Campaign to identify the future CMS sensor baseline. [2] J. Härkönen et al. (2008), Radiation hard silicon detectors for GSI/FAIR experiments [PPt slides]. [3] The Compact Muon Solenoid Experiment, http://cms.web.cern.ch/cms/Physics/Rewriting/index.html [4] A. Messineo – CERN (2011), Silicon Sensors Upgrade WG [5] Sentaurus Device User Guide, version F-2011.09, Synopsys